This invention relates to systems for measuring static attitude (orientation angle) of portions of head suspension assemblies.
Head suspension assemblies (xe2x80x9cHSAsxe2x80x9d) position a read/write head over the spinning surface of a data storage device (e.g. a magnetic hard disk). HSAs are some of the smallest and most delicate components of a rigid disk drive. An HSA includes a suspension assembly, an elongated spring structure, with a head assembly positioned at a distal end. Suspension assemblies act in a similar fashion to the needle arm in a record player, positioning the head assembly nanometers from the surface of a spinning disk in the disk drive. Typical suspension assemblies measure less than 20 mm long and are 0.03 to 0.1 mm thick. Suspension assemblies generally include an elongated load beam with a flexure located at a distal end and a base plate or other mounting structure located at a proximal end.
The flexure comprises a head bonding platform suspended by spring arms. The head assembly is mounted to this head bonding platform. The head assembly includes an air bearing slider and a read/write magnetic transducer formed on the slider. The slider is aerodynamically shaped to use the air stream generated by the spinning disk to produce a lift force. During operation of the disk drive, the spring arms provide gimballing motion to maintain the head assembly at a desired orientation with respect to the surface of the disk. The suspension assembly must balance the different lift forces on the outside and the inside air-bearing surfaces of the slider (the outside circumference of a round disk has a faster linear velocity than the inside, and therefore produces more lift), static forces (e.g. weight and pressure applied on the slider by the suspension assembly), and dynamic forces (e.g. momentum). The flexure and the whole HSA are manufactured within precise tolerances.
In a magnetic disk drive, the density and accuracy of the data stored on the disk depend on the distance (referred to as xe2x80x9cZ-heightxe2x80x9d) and orientation (referred to as xe2x80x9cstatic attitudexe2x80x9d) of the head assembly with respect to the surface of the disk. The size of the magnetic field xe2x80x9cspotxe2x80x9d written and read by the transducer is directly proportional to the square power of the Z-height distance between the transducer and the disk. Small changes in distance and/or attitude can cause the head assembly to xe2x80x9ccrashxe2x80x9d, that is, to hit the surface of the spinning disk. A crash can destroy both the transducer and the data on the surface of the disk. Tight manufacturing tolerances are a factor in determining disk drive reliability.
HSA manufacturers must repeatedly measure and control the Z-height and static attitude of different elements of the HSA at various points during the manufacturing process. The reference point for both the Z-height and the static attitude measurements is a manufacturing datum plane. The manufacturing datum plane is a horizontal plane representing a suspension mounting surface of an actuator. During manufacturing, the manufacturing datum plane is placed generally parallel to and below the suspension assembly.
A static attitude measurement includes a pitch axis angle measurement and a roll axis angle measurement measured in relation to the datum plane. The pitch and the roll axes are parallel to the horizontal plane and are mutually perpendicular, intersecting at a point on the head bonding platform. The roll axis is usually aligned with the longitudinal axis of the suspension assembly.
Static attitude can be measured using autocollimation systems. Autocollimation systems are measuring instruments that generate a collimated light beam (a light beam having parallel rays of light) having a relatively large diameter. The collimated light beam is directed to and reflected off of the surface of the part being measured. The reflected light beam strikes a linear array of light sensors. The sensors collect data on the reflected light beam which is fed into a computer to calculate the pitch and roll angles of the part. Autocollimation systems offer accurate and fast angle measurements.
In many instances, both the Z-height and the static attitude are measured by the same instrument in order to save time and space in the manufacturing process, as well as reduce errors in the measurement procedure. A single light source, such as a laser, produces a beam of light which is then split by a beam splitter into a first beam used to measure the Z-height and a second beam used to measure the static attitude. The second beam is directed by additional optics toward the head suspension target. This beam is then reflected back off the target and back to a detector.
As shown in FIG. 1, a prior art static attitude measurement system 20, such as a WYKO(trademark) SAT probe produced by Veeco Instruments Inc., Plainview, N.Y., includes a light source 30 to produce a light beam 32. Light beam 32 is typically linearly polarized at an angle, which is a combination of two components, one at a first polarization state (as represented by a solid line) and one at a second polarization state (as represented by a dashed line). System 20 also includes a beam splitter 40 that splits beam 32, passing a first beam 33 and reflecting a second beam 34. Due to the angle of incidence of beam 32 on beam splitter 40 and other reasons, the first beam 33 almost exclusively contains light in the second state of polarization, which is directed toward optics (not shown) and used for measuring Z-height of a suspension 64. The second beam 34, however, is a mixed beam and primarily includes light in the first state of polarization, but also some light in the second state of polarization. This second beam 34 is directed toward a second beam splitter 42. A beam reducer (not shown) may optionally be located between the first and second beam splitters 40, 42, in order to reduce the diameter of the beam 34.
The second beam splitter 42 splits the beam 34 by passing a beam 35 and directing a beam 36 orthogonally toward the suspension 64 to measure the attitude on a surface 62 of the suspension 64. Beam 36 is reflected off of the surface 62 and travels back in the opposite direction of the incoming beam. Beam 36 passes through some collection optics 44, such as a pair of lenses, when traveling toward and away from the suspension 64.
When returning beam 36 encounters the second beam splitter 42, a portion 38 of the beam 36 passes through and a portion is reflected back toward the first beam splitter 40. The passed portion 38 then encounters a series of optics 72, 74, 76 that filter out unwanted light, before the beam 38 strikes a detector 70. In particular, optics 76, such as a polarized filter, stops the second state polarized light portion in beam 38, but passes the primary portion in the first state of polarization. The information collected by the detector 70 is then used by a computer or other instrument to calculate the static attitude of the suspension 64.
Although the provided system is effective in producing static attitude measurements, there are some side effects to the optics that create problems with the system. As stated above, when the returning beam 36 encounters the second beam splitter 42, a portion of the beam is reflected back toward the first beam splitter 40. Because beam 36 primarily includes light at the first polarization state, the majority of this reflected, secondary beam is also at the first polarization state. If this secondary beam is strong enough, due to the surface 62 being highly reflective or other reasons, this beam may be reflected back toward the light source 30 by the first beam splitter 40. This beam then reflects off the light source 30, shown as phantom beam 52, which then is split at beam splitter 40 into beam 54, which is then split and directed by second beam splitter 42 toward the suspension 64 as beam 56. Beam 56 is then reflected off the surface 62 in a manner similar to that of beam 36. As a result, a portion 58 of reflected beam 56 is directed through the optics 72, 74, 76 to the detector 70, thereby causing a second measurement or stray spot at the detector 70. When the target being measured is highly reflective, this stray spot may be as bright as the primary spot, which can cause a substantial problem in the static attitude measurement of the suspension 64.
In addition, the collection optics 44 can cause an internal reflection resulting in a reflected beam 48 which may also reach the detector 70. If the flu reflectivity of the suspension 64 is low and the true reflected spot from beam 38 is not very bright, then the software controlling the detector 70 will not be able to differentiate between the true reflected spot and the internal reflection, and thus an erroneous measurement may occur. In addition, the internal reflection 48 and stray spot 58 may tend to be as bright or brighter than the actual measurement beam 38, and therefore cannot be easily filtered out without blocking out the desired measurement beam 38. Therefore, there is a need to reduce and/or eliminate the stray spot and internal reflection problems of the current static attitude measurement devices in order to improve the measurement capability of these devices.
The present invention is a static attitude measurement device for measuring the static attitude of a head suspension target while reducing measurement errors due to stray spots and internal reflections. The device includes a light source for producing a light bean, a beam splitter for directing a first portion of the light beam toward a target from which a reflected beam is returned, and a detector for detecting the reflected beam at a predetermined polarization state. A polarization component for producing the predetermined polarization state in the reflected beam, which is preferably a quarter-wave plate, is then positioned between the beam splitter and the target. As a result, the light beam passes through the quarter-wave plate before encountering the target and the reflected beam passes through the quarter-wave plate before encountering the detector. Such double passage results in the reflected beam being orthogonally polarized with respect to the original light beam, which is also the predetermined polarization state.
Unfortunately, as the reflected beam passes back through the beam splitter, a portion of this beam may be reflected back toward the light source, from which it may be reflected and directed again toward the target. However, when this stray beam makes a double passage through the quarter-wave plate, it is again orthogonally polarized and returns to the original polarization state of the light beam. As a result, it is not in the predetermined polarization state and is thus excluded from striking the detector and causing stray spot errors in the measurement device.
When the measurement device includes collection optics positioned between the beam splitter and the target, such as one or more lenses, internal reflections off of these collection optics may occur as the light beam passes through the collection optics. However, by positioning the quarter-wave plate between the collection optics and the target, the internal reflection beams do not pass through the quarter-wave plate on their way toward the detector. Therefore, the internal reflection beams are usually not in the predetermined polarization state and thus are excluded from striking the detector and causing errors in the measurement device.
Use of a polarizing beam splitter may also help to reduce stray spot and internal reflection errors. In addition, the positioning of a polarizer between the light source and the beam splitter helps limit the polarization state of the beam directed toward the target, thus also helping to reduce stray spot and internal reflection errors.
In a combined measurement device that measures both static attitude and Z-height of a head suspension target, the light source is split into two components having primarily opposite polarization states. A second quarter-wave plate may be positioned in line with the Z-height light beam portion before the target so that any scattered reflected Z-height light off of the target will end up passing through two quarter-wave plates. Therefore, the polarization state of this reflected Z-height light is changed so that it is not in the predetermined polarization state and will not cause errors in the static attitude measurement.
The present invention also includes a method of measuring the static attitude of a head suspension target while reducing measurement error due to stray spots, internal reflections and/or stray Z-height measurement light.